January is the firstmonth of the year in the Julian and Gregorian calendars, and one of seven Gregorian months with the length of 31 days. It is, on average, the coolest month of the year within most of the Northern Hemisphere and the warmest month of the year within most of the Southern Hemisphere, characterizing the onset of the Winter and Summer in both hemispheres respectively.

January is named for Janus (Ianuarius), the god of the doorway; the name has its beginnings in en:Roman mythology, coming from the Latin word for door (ianua) - January is the door to the year.